Conventionally, digital imagers, as noted above, are characterized by a linear voltage-to-light response, or transfer function; that is, the digital imager output voltage is approximately linearly related to light incident on the digital imager. Specifically, the output voltage transfer function is linearly proportional to the intensity of the light incident upon the digital imager. This linear transfer function can be characterized by a dynamic range. The dynamic range is defined as the difference between the highest detectable illumination intensity of a scene to the lowest detectable illumination intensity of the scene. It is well understood that the dynamic range of the transfer function sets the overall dynamic range of the digital imager. If the dynamic range of a scene exceeds the dynamic range of a digital imager, portions of the scene will saturate the digital imager and appear either completely black or completely white. This can be problematic for imaging large dynamic range scenes, such as outdoor scenes.
A conventional digital imager includes a plurality of photosensitive elements commonly referred to as pixels. The physical realization of the pixels is either a plurality of phototransistors or a plurality of photodiodes functioning as light-detecting elements. In operation, the pixel is first reset with a reset voltage that places an electronic charge across the capacitance associated with the diode. Electronic charge produced by, for example, a photodiode, when exposed to illumination, causes charge of the diode capacitance to dissipate in proportion to the incident illumination intensity of the scene. At the end of an exposure period, the change in diode capacitance charge is detected and the photodiode is reset. The amount of light detected by the photodiode is computed as the difference between the reset voltage and the voltage corresponding to the final diode capacitance charge.
The sensitivity of a digital imager, such as that provided in a digital still or video camera, can in general be defined by the maximum and minimum light intensities of a scene that can be imaged by the digital imager. The light or illumination intensity of a scene captured by a pixel of a digital imager is a value related to an output voltage or amount of charge measured at the pixel, photosensitive element.
There are various situations that can limit the sensitivity of a digital imager. One example is pixel saturation. Pixel saturation is realized when a photosensitive element, pixel, becomes depleted of charge such that no amount of additional incident light will change the output voltage, thus pixel saturation acts to limit the maximum light intensity of the scene that can be discretely imaged. In other words, if the saturation of a pixel is an illumination intensity level of 100, all illumination intensity levels of the scene that are above an illumination intensity level of 100 are represented as an illumination intensity level of 100 in the imaged scene, and thus, these illumination intensity levels of the scene that are above an illumination intensity level of 100 are not discretely imaged or are not discernible because notwithstanding the actual illumination intensity level, above 100, the imaged illumination intensity level is 100.
Another example is pixel noise. Examples of sources of pixel noise are quantization in the conversion of the analog voltage to a digital code, integrated thermal charge production, and voltage reset variation when a pixel is not sampled using a true correlated double sample. Pixel noise sets the minimum detectable illumination intensity. Since the human eye has a logarithmic response to intensity, in digital imaging, it is usually noise in the lower intensities that is of concern.
Both pixel saturation and pixel noise are inherent properties of a digital imager. So, these properties limit the physical sensitivity of a digital imager such that a scene, having a wide range of light (illumination) intensities, cannot be imaged properly; the higher light (illumination) intensities of the scene are washed out in the imaged scene due to pixel saturation, or the low light (illumination) intensities of the scene become less resolvable due to pixel noise.
Conventionally, methods have been proposed to nullify these physical limitations of a digital imager, such as the manipulation or adjustment of the charge integration function of the pixel, also known as the transfer control function of the digital imager. Charge integration function manipulation or transfer control function manipulation has, conventionally, been realized through the changing of an integration time, Tint, for the digital imager. Changing the integration time, Tint, changes the start time of the transfer control function or charge integration period. In other words, changing the integration time is a form of electronic-irising or exposure control, analogous to controlling the speed of a shutter on an analog camera. An example of conventional integration time manipulation is illustrated in FIGS. 1 and 2.
FIG. 1 shows a linear transfer control function wherein an integration time, Tint, is near a maximum integration time. Integration time, Tint, is the time that a control signal 10 is not set at a reset level. When the control signal 10 is not at a reset value during a frame period, F, the digital imager causes charge to be transferred or collected from a pixel. As shown in FIG. 1, the control signal 10 is initially at a reset level (at the beginning of the frame period, F); however, the control signal 10 changes to another level, in this case a collect level, at a point in time, within the frame period, that is equal to a difference between the frame period, F, and the integration time, Tint. (In FIG. 1, the difference is very small.) The control signal 10 changes back to the reset level at the end of the frame period, F.
On the other hand, FIG. 2 shows a linear transfer control function wherein the integration time, Tint, is decreased. As noted above, integration time, Tint, is the time that a control signal 10 is not set at a reset level. When the control signal 10 is not at a reset value during a frame period, F, the digital imager causes charge to be transferred or collected from a pixel. As shown in FIG. 2, the control signal 10 is initially at a reset level (at the beginning of the frame period, F); however, the control signal 10 changes to another level, in this case a collect level, at a point in time, within the frame period, that is equal to a difference between the frame period, F, and the integration time, Tint. (In FIG. 2, the difference is approximately equal to half the frame period, F.) The control signal 10 changes back to the reset level at the end of the frame period, F.
In other words, FIG. 2 shows a situation where the control signal 10 is set at a level that causes charge to be transferred or collected from the pixel for a shorter duration of time than shown in FIG. 1. By decreasing the integration time, the low illumination intensity levels of the scene being imaged are de-emphasized because not enough time is given to for the charge associated with the low illumination intensity levels of the scene to accumulate because the low illumination intensity level does produce charge as quickly as the high illumination intensity levels.
Another way of looking at the relationship of the integration time, Tint, of the digital imager to the illumination intensity level of the imaged scene is shown in FIG. 3. In FIG. 3, curve A represents a first mapping of pixel output voltage versus illumination intensity level of the imaged scene using a first integration time, T1int. Curve B shows a second mapping of pixel output voltage versus illumination intensity level of the imaged scene using a second integration time, T2int. In FIG. 3, the second integration time, T2int, is smaller than the first integration time, T1int. Thus, as illustrated in FIG. 3, as the integration time, T1int, is increased, the slope of the map of pixel output voltage versus illumination intensity level of the imaged scene is increased, thereby emphasizing the lower illumination intensity levels of the imaged scene.
Another conventional way of manipulating the transfer control function is to use a stepped or piecewise discrete-time transfer control function. By using a stepped or piecewise discrete transfer control function, the mapping of the pixel output voltage versus illumination intensity level of the imaged scene can be modified to enable a wider range of possible illumination intensity levels of the imaged scene before saturation, while emphasizing low illumination intensity levels of the imaged scene.
An example of a conventional stepped or piecewise discrete-time transfer control function is illustrated in FIG. 4. In FIG. 4, the integration time, T1int, is the same value as illustrated in FIG. 1. However, unlike FIG. 1, the level of the control signal 10, in FIG. 4, is stepped down in a discrete fashion from a reset level to a collect value. The control signal 10 acts as a barrier to charge production or accumulation. At the reset level, a complete barrier is realized wherein no charge is produced or accumulated, notwithstanding the illumination intensity level of the light incident upon the pixel. In other words, the pixel is effectively turned off. As the control signal 10, is stepped down from a reset level, the effective barrier is lower, thus allowing a rate of production or accumulation of charge to effectively realize a gradual increase.
Specifically, as illustrated in FIG. 4, over the course of an exposure period, the control signal 10 is applied to the pixel or photodiode capacitance to control charge dissipation from the capacitance. The control signal 10 is typically decreased from the starting pixel reset voltage value to, e.g., electrical ground or a collect level, with each control signal 10 value at a given time during the exposure period setting the maximum charge dissipation of the photodiode. This control signal 10 decrease acts to increase the photodiode charge dissipation capability, whereby the pixel can accommodate a higher illumination intensity before saturating, and the dynamic range of the pixel is thusly increased. This charge dissipation control produces a nonlinear pixel output voltage to imaged scene illumination intensity level mapping and a correspondingly expanded dynamic range of the pixel and the imager. This is seen in FIG. 5.
Using a step or piecewise discrete-time transfer control function, as illustrated in FIG. 4, a mapping of the pixel output voltage versus illumination intensity level of the imaged scene having a wider range, as illustrated in FIG. 5, is produced. In FIG. 5, curve A shows a mapping of the pixel output voltage versus illumination intensity level of the imaged scene when a linear control transfer function, as illustrated in FIG. 1, is used, whereas curve B shows a mapping of the pixel output voltage versus illumination intensity level of the imaged scene when a stepped or piecewise discrete-time transfer control function, as illustrated in FIG. 4, is used. As illustrated in FIG. 5, curve B has a wider range of illumination intensity levels of the imaged scene than curve A.
Although the physical limitations of digital imagers have been compensated for through integration time adjustment and transfer control function (resulting in the production of the control signal 10) manipulation, these conventional approaches still fail to compensate for the digital imager's physical limitations in an optimal way.
For example, the conventional processes fail to realize an illumination intensity level of the imaged scene range in which no saturated pixels exist. More specifically, as illustrated in FIG. 6, a histogram of actual illumination intensity levels of a scene to be imaged may include illumination intensity levels from 20 to 200 when the image average is 50; however, due to pixel saturation, illumination intensity levels of the scene above the 100 illumination intensity level will not be represented by the digital imager, as illustrated FIG. 7, because these higher illumination intensity levels of the scene are all bucketed at the 100 illumination intensity level by the digital imager due to pixel saturation, thereby washing out the higher illumination intensity levels of the imaged scene.
Using the conventional processes described above to compensate for the saturated pixels by reducing the integration time, a histogram of the illumination intensity levels of the imaged scene is realized as illustrated in FIG. 8. In FIG. 8, the histogram of the imaged scene is scaled by a factor of 0.5 such that the image illumination intensity average changes from 50 to 25. Using this conventional approach, a greater portion of the higher illumination intensity levels of the scene can be represented; however, the image is unacceptably dark, the quantization noise of the entire image has been cut in half, and the signal may also have been reduced by a factor of 2 while other noise sources stay fixed in magnitude (such as reset noise).